Nano Research & Applications

Description

Magnetic data storage has been pushing scientific innovations and technology limits for more than half a century and has grown into a multi-billion-dollar industry today. The recording of information in a magnetic medium, eg a thin magnetic film in a Hard Disk Drive (HDD), is achieved by switching its magnetization locally between two different orientations, which represent “1” and “0” of digitally stored information. The way of storing bit information in a granular medium with high magnetic anisotropy is by far the most cost-effective technology for high-density data storage and has a clear road map of extendibility up to 2 TB/in2. Such granular systems with high scalability and desired thermal stability can also be potentially leveraged into fabricating memory cells of non-volatile magnetic memory such as Magnetic Random Access Memory (MRAM). We thus envision a merging of hard drive granular medium and the MRAM concepts into a novel magnetic memory where reading and writing operations will be achieved by magneto resistive transport means like in MRAM, while the thermally stable perpendicularly magnetized grains with diameters of below 10 nm provide extraordinary scaling potential for memory applications.

Influence of an Externally Applied Magnetic Field

In this letter, we explore magneto resistive effects in the state-of-the-art perpendicular magnetic medium of today’s HDDs – granular oxide-segregated CoPtCr films deposited on glass substrates. We use point-contact technique to characterize the local transport in the film composed of granular layers with different cervicitis. When the relative orientation of layers changes under the influence of an externally applied magnetic field, we observe large variations in the point-contact resistance. Interestingly, the magneto resistance is essentially absent on the macroscopic scale – bulk resistivity measurements performed on the same films showed practically no magneto resistance. The observed local and large magneto resistance was found to decrease with the value of dc bias applied to the contact. Such variations are consistent with a tunnel magneto resistance between individual CoPtCr grains and support the potential of the granular medium for future magnetic memory technology. In our experiments we have tested a number of thin-film samples deposited by the media development team at Fremont Research Center, Seagate Technology with various layer combinations, compositions, and individual layer thicknesses ranging from 0 to 100 where all numbers in brackets are thickness in Å; this multilayer sample has a 63 nm thick adhesion/seed layer underneath, and a 6 nm thick continuous CoPtX capping layer (X is combination of non-magnetic diluting elements such as Cr, B, and Ru). The columnar structure is achieved by an epitaxy-like sputtering of a Ru-rich columnar growth template. Spacer layers are Ru-rich materials with oxide contents and are inserted to break the interlayer exchange coupling between adjacent ferromagnetic layers. Individual magnetic grains are segregated by a mixture of TiO2, SiO2 and other oxides.  a top-down planar-view Transmission Electron Microscopy (TEM) image of the granular composite media. The Magneto-Optical Kerr Effect (MOKE) hysteresis loop of a single magnetic layer of the granular medium, the Kerr signal confirms the perpendicular anisotropy of the material and displays the effects of the magnetic grain switching field distribution and demagnetizing field anisotropy as evidenced by the shearing of the major hysteresis loop. a result of micro magnetic modeling which simulates the hysteresis behavior of such a granular system. The modeling parameters were setup as follows: 656 voronoi grains are generated, with the mean and coefficient of variation of grain-to-grain pitch being 7.8 nm and 16.7%, respectively. The 0 K saturation magnetization and anisotropy field of the medium are 650 emu/ cc and 9.2 kOe. Demagnetization and thermal effects (300 K) were taken into account during the simulation. The damping constant is set to be 0.2 in order to reach equilibrium state within a reasonable computational time. The modeling shows that there are various portions of grains that get reversed at low, median, and high fields, indicating nucleation, coercively, and saturation, respectively.

Anisotropy Granular Medium and Point-Contact

Magneto resistive properties of bulk films were assessed by measuring the film resistance in the Van der Pauw geometry: four-point probes (wire-bonds) were placed in the corners of square-shaped media film. A manual ultrasonic West-Bond wire bonder was used to make the probes; Keithley 2400 source meter and keithley 2000 digital multimeter were used to source current and measure voltage, respectively. An external magnetic field could be applied perpendicular to the film’s plane (FPP-orientation) or in the plane of the film (FIP-orientation). Local magneto transport properties of the same thin-film samples were assessed by point contacts used as a probe of small sample volumes typically associated with such contacts. The point contact was made by bringing a sharpened metallic (Cu or W) tip into contact with the multilayer using a differential screw mechanism; details of the point-contact setup can be found here. This mechanical point-contact technique enables us to produce electrical contacts of only a few nanometers in diameter (a) and probe electrical transport in very small sample volumes. In this work we focus on dc transport measurements of point contacts in magnetic fields B up to 0.7 T applied in the FPP-orientation and dc biases I up to 1.5 mA. All measurements were done at room temperature. Contrary to the bulk measurements, point-contact measurements with Cu tips revealed a larger (~1%) MR in both the FPP and FIP orientations small variations in the point-contact resistance under a varying magnetic field confirm that the magneto strictive effect at the interface between the high anisotropy granular medium and point-contact tip is negligible. At the same time, the relative softness of the copper tip material compared to the hard media sample can be at the origin of a relatively small MR ratio. In order to make stable electrical contact with the sample, a high amount of pressure visibly deformed sharpened Cu tips thus resulting in large contact areas and, consequently, involving many individual grains into the contact region. Due to the complications associated with the relatively soft copper tips, we have switched to tungsten as the point-contact tip material for better mechanical robustness.